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A novel core–shell species for the adsorption-based separation of carbon dioxide (
CO2) from methane (
CH4) is introduced
by hydrothermal synthesis of Ni-MOF-74 on mesoporous spherical Al2O3
carrier substrate. The material was characterized
and the shell thickness determined by means of optical and scanning electron microscopy as well as volumetric
adsorption and fluid displacement experiments. Kinetic experiments with Ni-MOF-74@Al2O3 core–shell composites carried
out at 303.15 K and at pressures up to 10 bar expose remarkably dominating uptake rates for CO2
over CH4.
In the
contrary Ni-MOF-74@Al2O3 appears to be unselective according to equilibrium data at the same conditions. Dynamic
breakthrough experiments of binary CH4/
CO2-mixtures (at 303.15 K and 5 bar) prove the prevailing effect of adsorption
kinetics and the storage function of the mesoporous core. This statement is supported by a considerable boost in
CO2-
selectivity and capacity compared to adsorption equilibria measured on pure Ni-MOF-74 by the factor of 55.02 and
up to 2.42, respectively.
Sorption measurements of water vapor on an isoreticular series of Imidazolate Frameworks
Potsdam (IFP), based on penta-coordinated metal centers with secondary building units (SBUs)
connected by multidentate amido-imidate-imidazolate linkers, have been carried out at 303.15 K. The
isotherm shapes were analyzed in order to gain insight into material properties and compared to
sorption experiments with nitrogen at 77.4 K and carbon dioxide at 273.15 K. Results show that water
vapor sorption measurements are strongly influenced by the pore size distribution while having a
distinct hysteresis loop between the adsorption and desorption branch in common. Thus, IFP-4 and
-8, which solely contain micropores, exhibit H4 (type I) isotherm shapes, while those of IFP-1, -2 and
-5, which also contain mesopores, are of H3 (type IV) shape with three inflection points. The choice
of the used linker substituents and transition metals employed in the framework has a tremendous
effect on the material properties and functionality. The water uptake capacities of the examined IFPs
are ranging 0.48 mmol g????1 (IFP-4) to 6.99 mmol g????1 (IFP-5) and comparable to those documented for
ZIFs. The water vapor stability of IFPs is high, with the exception of IFP-8.
Based on experimental pure component data for the characterization of the isostructural imidazolate framework Potsdam (IFP) series reported in Part I, a model for the simulation of non-isothermal dynamic adsorption of CO2/CH4-mixtures in fixed-bed columns is presented in this Part II. The robustness of the model is examined and validated, by comparison to experimental breakthrough data at different process conditions, such as varying concentration, temperature, and pressure. Thereby, different predictive methods for the estimation of adsorption equilibria of mixtures are compared (RAST, IAST, ML). The results show that ideal behaviour can be assumed with good accuracy for the system under consideration, except for IFP-2, which shows significant deviations at increased pressures and temperatures. A detailed kinetic analysis reveals that mass transfer is significantly influenced by micropore diffusion. Thus, only for IFP-1 the dynamic separation of CO2 and CH4 is equilibrium-driven, while for the remaining IFPs the kinetic regime dominates the process, which in some cases increases the separation efficiency (IFP-2 to -7) but can also inhibit it (IFP-8). The determined intracrystalline diffusion coefficients show very good agreement with values for metal organic framework (MOF) compounds of similar structure reported in the literature.
A novel core–shell species for the adsorption-based separation of carbon dioxide (CO2) from methane (CH4) is introduced by hydrothermal synthesis of Ni-MOF-74 on mesoporous spherical Al2O3 carrier substrate. The material was characterized and the shell thickness determined by means of optical and scanning electron microscopy as well as volumetric adsorption and fluid displacement experiments. Kinetic experiments with Ni-MOF-74@Al2O3 core–shell composites carried out at 303.15 K and at pressures up to 10 bar expose remarkably dominating uptake rates for CO2 over CH4. In the contrary Ni-MOF-74@Al2O3 appears to be unselective according to equilibrium data at the same conditions. Dynamic breakthrough experiments of binary CH4/CO2-mixtures (at 303.15 K and 5 bar) prove the prevailing effect of adsorption kinetics and the storage function of the mesoporous core. This statement is supported by a considerable boost in CO2-selectivity and capacity compared to adsorption equilibria measured on pure Ni-MOF-74 by the factor of 55.02 and up to 2.42, respectively.